[Neural regulation of heart rate. Spectrum analysis of the RR interval in healthy subjects].

Dynamic 24-hour recordings were obtained in 10 healthy non hospitalized subjects. Spectral analysis of RR interval variabilities provided quantitative markers of sympatho-vagal balance throughout the day and night. The low frequency (0.1 Hz) component was considered a marker of sympathetic activity whereas the high frequency component (0.25 Hz) a marker of vagal tone. We observed an early morning rise in the sympathetic activity markers in all our patients, with a second smaller rise in the early afternoon. We believe this rise may be the trigger of the increased rate of major cardiovascular events reported to happen in the first hours of the morning.